1. Field of the Invention
The present invention relates to a noise filter which is suitable for use in suppression of electromagnetic noise interferences in electronic devices.
2. Description of the Related Art
In general, various types of noise filters are used for suppression of electromagnetic noise interferences in electronic devices. In a known related art noise filter, for example, a circuit using a lumped constant such as a three-terminal capacitor is formed (see for example, Japanese Unexamined Patent Application Publication No. 10-154632). In such a noise filter of the related art, noise is suppressed using a reflection loss in which the reflection coefficient is increased at the frequency of noise to be generated.
Moreover, in a known noise filter according to another related art device, a circuit including many inductors and capacitors is formed (see for example, Japanese Unexamined Patent Application Publication No. 2000-34894).
According to the above-mentioned related art techniques, noise is suppressed by use of the reflection loss. In the case where a noise filter is disposed, e.g., in a line connecting circuits to each other, problems are caused in that probably, noise having a specific frequency resonates between the noise filter and a circuit in the periphery thereof, and the resonance causes amplification of the noise.
Especially, in recent years, signal frequencies to be adapted for digital devices tend to become higher, and the number of electronic devices of which the signal frequencies exceed 100 MHz has been increased. Therefore, low-pass filters of which the cut-off frequencies exceed 200 MHz have been demanded. On the other hand, for example, the lengths of lines between a noise filter and components disposed at the periphery thereof and the lengths of lines between plural components are such that resonance with a signal (noise) with a high frequency of 200 MHz or higher may occur easily. Accordingly, for electronic components of which the signal frequencies exceed 100 MHz, noise filters using reflection loss as carried out in the related art devices are difficult to be used.
Moreover, according to the other related art device, resonance phenomena are suppressed by reduction of the reflection loss. However, in the noise filter of the other related art, many inductors and capacitors are connected to form a circuit. The structure is complicated, the size can be reduced with much difficulty, and the manufacturing cost is high. Moreover, problematically, the noise filter is difficult to be fixed to a printed wiring, since the filter does not have a chip-shape configuration.
In order to overcome the problems described above, preferred embodiments of the present invention provide a noise filter that prevents noise from resonating and has a very small size and is inexpensive.
According to a preferred embodiment of the present invention, a noise filter includes a plurality of magnetic sheets stacked on one another; transmission lines, and ground conductors. The ground conductors are arranged on the top and bottom layers of the magnetic sheets. The transmission lines and the ground conductors are alternately stacked between the magnetic sheets. One end of each of the transmission lines is connected to a different corresponding signal input electrode, and the other end of each of the transmission lines is connected to a different one of the corresponding signal output electrodes, and the transmission lines of the multiple layers have different characteristic impedances.
Since the transmission lines of the multiple layers are connected to the different corresponding signal input electrodes and signal output electrodes, the transmission lines of the multiple layers individually function as low-pass filters, and the entirety of the transmission lines serves as a noise filter array. Noise is suppressed by utilizing an increase in heat dissipation by the magnetic sheets when RF signals pass through the transmission lines of the layers. The characteristic impedance of the noise filter is set by setting the width dimensions of the transmission lines and the thickness dimensions of the magnetic sheets to appropriate values. In particular, the relative magnetic permeability of a magnetic material is a constant irrespective of the signal frequency, the impedance is matched in substantially the entire frequency domain with a circuit connected to the noise filter, and return loss of the noise filter is reduced.
The transmission lines of the multiple layers individually function as independent low-pass filters. When, for example, the transmission lines of the multiple layers are connected using through lines penetrating through the magnetic sheets, impedance mismatching easily occurs near the through lines, which are discontinuous points. Compared with such a case, according to preferred embodiments of the present invention, impedance mismatching does not occur in the transmission lines. Therefore, noise reflection does not occur in the transmission lines, noise resonance is suppressed, and impedance matching with an external circuit is easily achieved.
While the ground conductors are arranged on the top and bottom layers of the plural magnetic sheets which are stacked on one another, the transmission lines and the ground conductors are alternately disposed between the magnetic sheets. Accordingly, the transmission line of each layer is held between the two magnetic sheets, and the entire transmission line of each layer is covered by the two ground conductors. Thus, a signal passing through the transmission line of each layer is confined between the ground conductors, and a pass-band signal is prevented from attenuating.
Since the ground conductors are arranged on the top and bottom layers of the plural magnetic sheets superposed on one another, external noise is prevented from entering the transmission lines of the layers. Thus, signals are reliably transmitted.
The transmission lines of the multiple layers have different characteristic impedances. Accordingly, the transmission lines of the layers are connected to wiring having a plurality of types of characteristic impedances while the impedances of the transmission lines are matched with those of the wiring. By connecting the transmission lines in parallel with one another, constituting some or all of the multiple layers, the number of types of characteristic impedances is increased. As a result, the number of types of wiring to which the noise filter is applicable is increased.
The transmission lines may be meandering and/or zigzag. Alternatively, the transmission lines may be spiral. Accordingly, compared with a case in which the transmission lines are linear, the length dimensions of the transmission lines are increased, and noise attenuation is increased.
According to another preferred embodiment of the present invention, a noise filter includes magnetic sheets, a transmission line arranged on the front surface of one of the magnetic sheets, and a ground conductor arranged on the back surface of the one magnetic sheet. The ground conductor covers, from the back, the entirety of the transmission line.
With this arrangement, noise in a signal passing through the transmission line is suppressed by utilizing heat dissipation by the magnetic sheets. The characteristic impedance of the noise filter is set by setting the width dimension of the transmission line and the thickness dimensions of the magnetic sheets to appropriate values. The characteristic impedance is maintained substantially at a constant irrespective of the signal frequency. The impedance is matched in substantially the entire frequency domain with a circuit connected to the noise filter. Therefore, return loss of the noise filter is reduced. Since the entire transmission line is covered, from the back, by the ground conductor, the characteristic impedance of the entire transmission line is set to a constant. Therefore, noise reflection and noise resonance are suppressed.
The transmission line may be spiral. Accordingly, magnetic fluxes generated by the transmission line are added together. Thus, the inductance and the characteristic impedance of the transmission line are increased.
The transmission line may be meandering and/or zigzag. Accordingly, compared with a case in which the transmission line is linear, the length dimension of the transmission line is increased, and noise attenuation is increased.
The magnetic sheets may be substantially rectangular. Signal electrodes connected to both ends of the transmission line may be arranged adjacent to both ends, in the longitudinal direction, of the one magnetic sheet. Grounding electrodes connected to the ground conductor may be arranged in the middle positions, in the longitudinal direction, of the one magnetic sheet.
Accordingly, the signal electrodes placed adjacent to both ends, in the longitudinal direction, of the magnetic sheet are easily connected in the middle of linear wiring. The grounding electrodes, which are placed in the middle positions in the longitudinal direction of the magnetic sheet, are easily connected to ground terminals arranged in the vicinity of the wiring. It thus becomes easier to assemble the noise filter.
The magnetic sheets may be made of sintered ferrite or other suitable material.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.